Basic SPI features like clock phase/polarity and loopback mode are
common to all RSPI variants.  Factor them out to reduce duplication.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200218105810.902-2-geert+renesas@glider.beSigned-off-by: NMark Brown <broonie@kernel.org>